Process control strip and method for recording

ABSTRACT

A process control strip for visual monitoring of an exposure process for a recording material as course signal elements and fine signal elements. A first stripe extending in a direction of a greatest expanse of the process control strip has a tonal value wedge with process-independent reference tonal values as the course signal elements that change in the stripe direction. A second stripe proceeds parallel to the first stripe and has a raster with fine raster points and the fine signal elements that represent a uniform, highly process-dependent tonal value.

BACKGROUND OF THE INVENTION

The invention is in the field of electronic reproduction technology andis directed to a process control strip for visual monitoring andcalibration of an exposure process for a recording material,particularly for a printing plate, and is also directed to a method forrecording the process control strip.

The point-by-point and line-by-line, rastered exposure of a recordingmaterial, for example a film, usually occurs with an electronicrecording device, also called an exposer or recorder. For that purpose,image signal values that represent the tonal values to be recorded aresupplied to a raster generator in which the image signal values areconverted according to a raster function into control signal values foran exposure beam generated in an exposure unit of the exposer. Thepixel-by-pixel and line-by-line exposure of the film occurs during arelative motion between the exposure beam and the film to be exposed inthat the control signal values turn the exposure beam on and off andthus determine which pixels are exposed as parts of the raster points onthe film or are not exposed. The raster function thereby determines thesize of the raster points dependent on the tonal values to be recorded.

In the exposure of the film, the real tonal values or, raster pointsizes deviate from the desired, nominal tonal values since every pixeland, thus, every raster point is recorded more or less enlarged due toblooming. The deviations between the tonal values that are reallygenerated and the nominal tonal values are referred to as point growthsthat lead to disturbing changes in tonal value in the reproduction.

The point growths are thereby compensated in the exposer during the filmexposure in that the image signal values that represent the nominaltonal values are corrected by what is referred to as a filmlinearization according to a correction curve determined before the filmexposure such that the tonal values really recorded on the filmcorrespond to the nominal tonal values.

After the film exposure, the film exposed in the exposer is developed ina developing station and is used for manufacturing a printing form.

The traditional manufacture of printing plates occurs in twosub-processes. In a first sub-process, a film is exposed with an exposerand the exposed film is developed in a developing station. In a secondsub-process, the exposed and developed film, as a master, is copied ontoa light-sensitive printing plate in a copier device, whereby slightpositive or negative point growths and, thus, falsifications of tonalvalue can likewise occur. After the copying process, the exposedprinting plate is then likewise developed in a developing station.

Calibrations, i.e. settings and checks of the optimum processparameters, corresponding to two sub-processes must thus be undertakenin the traditional manufacture of a printing plate.

The traditional calibration of the first sub-process, namely thepoint-by-point and line-by-line film exposure in an exposure and thefilm developing in a developing station, occurs, for example, with theassistance of graduated standardized step wedges that are exposed on thefilm and co-developed, and via the measurement of the full-tinedensities. A constant monitoring of the stability of exposure anddevelopment is also involved in practice with the known means. For thisreason, adhering to a stable work process has previously occuredindirectly by monitoring and by controlling or, setting suitable processparameters such as the intensity of the exposure beam and/or thecorrection curve in the exposer as well as the development temperatureand/or the regeneration rates in the developing station.

The traditional calibration of the second sub-process, namely theimage-wise exposure of the printing plate in a copier device and thedevelopment of the exposed printing plate in a developing station, oftenoccurs according to the micro-line method with the assistance ofprecision measuring strips, for example with the FOGRA precisionmeasuring strip PMS-I or the UGRA Offset Test Wedge 1982. Theseprecision measuring strips are described in detail in, for example, the"Fogra Praxis Report" No. 34, 1990, Fogra-PMS-I and UGRA-Offset-Testkeil1982 (FOGRA=Deutsche Forschungsgesellschaft fur Druck- undReproduktionstechnik e.V.).

DE-A-23 56 325 discloses a test film that is copied onto a printingplate in a copier device together with the master in order to generate acontrol image for visual monitoring of the following developmentprocess. The test film comprises fine signal elements in the form offinely structured zones and coarse signal elements in the form of acoarsely structure background zone that surrounds the finely structuredzones and separates them from one another. The zones are respectivelycomposed of a plurality of points. The finely structured zones are ofsuch a nature that a modification of the process conditions leads to avisible change in their optical density, whereas the optical density ofthe coarsely structured background zone changes only slightly givenmodification of the process conditions, modifications in the processconditions being thus visually displayed.

A constant monitoring of the stability of the copying process anddevelopment of the printing plate is likewise also involved in practicewith the known means. For this reason, adherence to a stable workprocess has also previously occurred indirectly by monitoring and bycontrolling or, setting suitable process parameters such as, forexample, the exposure duration or, the numbers of clocks and theduration of the vacuum suctioning of the printing plate in theimage-wise exposures in the copying device as well as the developmenttemperature or the regeneration rates in the developing station. Forreasons of expense, these process parameters are often only checked atgreater time intervals, usually in conjunction with new batches ofmaterial.

There is currently a trend in reproduction technology to not produce theprinting plates in two sub-processes via the intermediate medium offilm, but to directly expose them in an exposer (computer-to-plate).Since the calibration and control methods with the assistance of theknown process control strips are based on the intermediate medium offilm, they cannot be applied in the direct exposure of printing platesin an exposure. Over and above this, the calibration and control methodsimplemented with the known process control strips have the disadvantagethat they require measuring aids and practically do not allow a simple,continuous process monitoring.

SUMMARY OF THE INVENTION

It is therefore object of the invention to improve a process controlstrip for the visual monitoring and calibration of an exposure processfor a recording material, particularly for a printing plate, as well asa method for recording the process control strip such that it can alsobe applied in the direct exposure of printing plates in electronicrecording devices and thereby enable a high-grade quality monitoringwith respect to exposure and development.

According to the invention, a process control strip is provided forvisual monitoring of an exposure process for recording material. Coarsesignal elements having a size substantially constant given processfluctuations are provided along with fine signal elements having a sizewhich changes given process fluctuations. A first strip extending in adirection of a greatest expanse of the process control strip and havinga tonal value wedge with process-independent reference tonal values isprovided as said course signal elements that change in the stripdirection. A second strip is provided parallel to the first strip andhaving a raster with fine raster points as said fine signal elementsthat represent a uniform, highly processed-dependent tonal value.

The invention is described in greater detail below on the basis of FIGS.1 through 4.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structure of a process control strip for thedirect exposure of printing plates with an exposer;

FIG. 2 is a practical exemplary embodiment of a process control strip;

FIG. 3 is a process control strip simulated as a contone print; and

FIG. 4 is a schematic block circuit diagram of an apparatus for thedirect exposure of printing plates.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the schematic structure of a process control strip 1 forthe direct exposure of printing plates with an exposure(computer-to-plate).

During the direct exposure of the printing plate in the exposer, theprocess control strip 1 is exposed onto the printing plate outside theprinting plate region provided for the information to be exposed and isdeveloped together with the information in a developing station. Theexposed and developed process control strip 1 serves for the visualmonitoring and setting of the process parameters, such as the intensityof the exposure beam as well as the development temperature and/or theregeneration rates in the developing station.

The process control strip 1 is basically composed of three stripesarranged parallel to one another that extend in the direction of thegreatest expanse of the process control strip 1, namely a rated valuestripe 2, an actual value stripe 3 and a display stripe 4.

In the exemplary embodiment, the rated value stripe is a graduated tonalvalue wedge with, for example, 16 reference tonal value steps from 0%through 100%. The reference tonal values are process-independent to thefarthest-reaching extent, i.e. they change only insignificantly givenfluctuations of process parameters.

A rated value region 5 that contains at least one reference tonal valuestep as a rated value range of tolerance that is to be achieved on theprinting plate in the exposure and development process can be definedwithin the tonal value wedge of the rated value stripe 1. The referencetonal value steps of the tonal value wedge are thereby expedientlyselected such that the desired rated value region 5 lies in the middleregion of the process control strip 1.

Instead of a tonal value wedge with graduated reference tonal values, atonal value wedge with continuously varying reference tonal values canalso be employed.

The tonal value wedge of the rated value stripe 2 is designed as a lineraster with lines 6 oriented perpendicular to the expanse of the processcontrol strip 1 that are composed of individual pixels in the exposure.The reference tonal values of the tonal value wedge are defined by theratio of line width to line interval of the line raster. The lines 6 ofthe tonal value wedge represent coarse signal elements. The size of thecoarse signal elements changes only slightly, given fluctuations of theprocess parameters since the process-dependent changes of the pixelsizes lead to negligible changes in tonal value essentially only in theline direction at the lateral edges of the lines 6, as a result whereofthe reference tonal values of the rated value stripe 2 are essentiallyprocess-independent.

The structure of the line raster of the rated value stripe 2 is limitedby the resolution of the human eye and should be selected such that theintegrating effect with respect to a uniform impression is not lost. Afavorable value for the line spacings in the line raster lies in therange of 10 to 16 times the value of the pixel diameter that can be setby the addressing in the generation of the raster point.

The actual value stripe 3 proceeding parallel to the rated value stripe2 is finely rastered with 333 lines/cm and represents a highlyprocess-dependent but uniform tonal value within the actual value stripe3. The actual value stripe 3 is composed of a plurality of fine rasterpoints arranged in a raster, whereby each raster point within a rastermesh of the raster is composed of individual, exposed pixels in theexposure. The sum of the exposed pixel areas or, raster point sizeswithin a raster mesh referred to the total area of the raster meshdetermines the exposed tonal value. The exposed pixels or, the rasterpoints composed of the exposed pixels within the actual value stripe 3form fine signal elements whose size changes given fluctuations of theprocess parameters, as a result of which process-dependent tonal valuechanges arise.

In order to achieve pronounced tonal value changes, each raster point isexpediently composed of a comparatively great number of the pixelsavailable within a raster mesh of the raster, for example of 2×2 exposedpixels within a raster mesh constructed of 3×3 pixels. Aprocess-dependent modification of pixel size thus effects acomparatively great modification of the percentage area share in thetotal area of a raster mesh, so that pronounced changes in tonal valuewithin the actual value stripe 3 arise given modifications of pixel sizedue to fluctuations of the process parameters.

The structure of the raster in the actual value stripe 3 with respect tothe size of the raster mesh, the raster point size and the raster pointshape is limited by the resolution of the printing plate to be exposedand is thus dependent on the plate type and additionally is alsodependent on the addressing in the raster point generation. Practicalvalues are 3 to 5 times the addressing for the side length of a rastermesh assumed to be quadratic.

Each pixel size or, respectively, raster point size exposed on theactual value stripe 3 of the process control strip 1 thus represents atonal value achieved in the exposure process that coincides with areference tonal value of the tonal value wedge of the rated value stripe2.

The nominal condition for the exposure process is met when the tonalvalue achieved in the actual value stripe 3 falls in the defined ratedvalue region 5 of the rated value stripe 2.

When the process parameters change, then the tonal value of the actualvalue stripe 3 changes, whereas the tonal values of the tonal valuewedge in the rated value stripe 2 of the process control strip 1 remainpractically stable. Given a change of the process parameters, thecoincidence of the tonal values occurs at a different location of theprocess control stripe 1.

For simple visual checking of the degree or tonal value coincidence, theprocess control strip 1 comprises a display stripe 4 proceeding parallelto the rated value stripe 2 and the actual value stripe 3 that issubdivided into display fields 7 that are labeled with symbols and arearranged following one another in the longitudinal direction of thestrip. A display field 7a with the label, for example, "rated valueachieved" or "correct exposure" is thereby allocated to the definedrated value region 5 of the rated value stripe 2, whereas theneighboring display fields 7b, 7c are provided with the label "fallsbelow rated value" or "too little exposure" or, "exceeds rated value" or"too much exposure". In this way, one advantageously obtains alocation-dependent statement on the basis of the process control strip 1as to whether the printing plate is correctly exposed, underexposed oroverexposed.

FIG. 2 shows a practical exemplary embodiment for a process controlstrip 1 that, for example, is shown with 1000 lines/cm and was printedwith 300 dpi (dpi=dot per inch)

FIG. 3 shows a process control strip 1 simulated as contone print. Sincethe reproduction of the real optical impression is not possible forreasons of printing technology, the real optical impression is simulatedin FIG. 3 with a contone print of the process control strip 1.

When the calibration and monitoring method with the assistance of theprocess control strip 1 is used in the primary determination of theoperating point, i.e. in the process calibration, then the visual tonalvalue comparison advantageously supplies a continuous statement aboutthe process stability. The distance between the "coarseness" of the lineraster of the tonal value wedge in the rated value stripe 2 and the"fineness" of the point raster in the actual value stripe 3 therebydefines the sensitivity of the monitoring method.

The calibration and monitoring method with the assistance of the processcontrol strip 1 enables a high-sensitivity quality evaluation of theoverall process of direct exposure and development of printing plates.In particular, the high sensitivity assures the enhanced quality demandsthat are present in the exposure of printing plates withfrequency-modulated rasters.

FIG. 4 shows a schematic block circuit diagram of an apparatus fordirect exposure of printing plates, particularly offset printing plates.The apparatus is essentially composed of a raster image processor 8,simply referred to as an RIP, of a plate exposer 9 and of a platedeveloping station 10.

A printing sheet to be exposed on the printing plate and the processcontrol strip 1 to be exposed next to the printing sheet are therebyassembled, for example, in an electronic assembly station according toan imposition program. The PostScript image data thereby acquired arethen converted into a display list in an interpreter contained in theraster image processor 8. In a raster generator that is likewisecontained in the raster image processor, the display list is convertedaccording to a raster function into corresponding control signal valuesin the form of a bitmap for the pixel-by-pixel activation anddeactivation of an exposure beam generated in an exposure unit of theplate exposer 9.

The plate exposure 9 undertakes the pixel-by-pixel and line-by-lineexposure of the printing plate 11. During the plate exposure, thecontrol signal values of the bitmap determine which pixels are exposedas parts of the raster points or are not exposed on the printing plate11. The raster function thereby determines the size of the raster pointsdependent on the tonal values to be recorded. The exposure beam, forexample, is a laser beam that is switched on and off with a modulatorcontrolled by the control signal values. For example, the plate exposer"Gutenberg" of Linotype-Hell AG can be utilized as plate exposer 9.

The exposed printing sheet 12 and the process control strip 1 exposedoutside the printing sheet 12 are visible on the printing plate 11exposed in the plate exposer 9. For example, a CTX printing plate of thePolychrome company can be employed as a printing plate.

The exposed printing plate 11 is developed in the plate developingstation 10. The process control strip 1 on the exposed and developedprinting plate 11' is then employed for visual monitoring of theexposure process and for setting the process parameters.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that my wish is toinclude within the claims of the patent warranted hereon all suchchanges and modifications as reasonably come within my contribution tothe art.

I claim as my invention:
 1. A system for visual monitoring of anexposure process for a recording material, comprising:a process controlstrip having coarse signal elements having a size substantially constantgiven process fluctuations, and fine signal elements having a size whichchanges given process fluctuations; said control strip having a firststripe extending in a direction of a greatest expanse of the processcontrol strip and having a tonal value wedge with process-independentreference tonal values as said coarse signal elements that change in thestripe direction; said control strip having a second stripe proceedingparallel to the first stripe and having a raster with fine raster pointsas said fine signal elements that represent a uniform, highlyprocess-dependent tonal value; and an exposer for exposing the controlstrip on the recording material.
 2. The system according to claim 1wherein the tonal value wedge of the first stripe is designed as a lineraster.
 3. The system according to claim 1 wherein lines of the lineraster in the first stripe are oriented perpendicular to the stripedirection.
 4. The system according to claim 1 wherein lines of the lineraster in the first stripe and the raster points in the second stripeare formed of recorded pixels.
 5. The system according to claim 1wherein each raster point within a raster mesh of the raster of thesecond stripe is exposed from a great number of pixels available withinthe raster mesh.
 6. The system according to claim 5, wherein each rastermesh is formed of 3×3 pixels and each raster point within the rastermesh is formed of 2×2 pixels.
 7. The system according to claim 1 whereina rated value region that comprises at least one reference tonal valuedesired in the exposure process is defined in the tonal value wedge ofthe first stripe for visual comparison to the tonal value of the secondstripe achieved in the exposure process.
 8. The system according toclaim 1 wherein reference tonal values of the tonal value wedge in thefirst stripe are selected such that the defined rated value region liesin a middle region of the process control strip.
 9. The system accordingto claim 1 further comprising a third stripe proceeding parallel to thefirst and second stripes for displaying a degree of tonal valuecoincidence between reference tonal values of the first stripe and tonalvalues of the second stripe achieved in the exposure process.
 10. Thesystem according to claim 9 wherein the third stripe is subdivided intodisplay fields arranged following one another in the stripe directionthat indicate with symbols a respective degree of tonal valuecoincidence.
 11. The system according to claim 9 wherein:a display fieldin the third stripe having a symbol "rated value achieved" is allocatedto a defined rated value region of the first stripe; and neighboringdisplay fields of the third stripe are provided with symbols "ratedvalue exceeded" or "below rated value".
 12. The system according toclaim 1 wherein the recording material is a printing plate.
 13. A methodfor visual monitoring of an exposure process for a recording material,comprising the steps of:creating a process control strip having coarsesignal elements having a size substantially constant given processfluctuations, and fine signal elements having a size which changes givenprocess fluctuations; providing the process control strip with a firststripe extending in a direction of a greatest expanse of the processcontrol strip and having a tonal value wedge with process-independentreference tonal values as said coarse signal elements that change in thestripe direction; providing the process control strip with the secondstripe proceeding parallel to the first stripe and having a raster withfine raster points as said fine signal elements that represent theuniform, highly process-dependent tonal value; exposing the processcontrol strip on the recording material; and utilizing the exposedprocess control strip to monitor the exposure process.
 14. The methodaccording to claim 13 including the step of exposing the process controlstrip pixel-by-pixel and line-by-line directly on a printing plate. 15.The method according to claim 14 including the step of implementing theexposure of the process control strip simultaneously with thepoint-by-point and line-by-line exposure of the printing plate.
 16. Themethod according to claim 14 including the step of generating theprocess control strip as Post Script data.
 17. The method according toclaim 14 including the step of orienting the process control strip inthe point-by-point and line-by-line exposure of the printing plate suchthat the lines of the line raster in the first stripe proceed in theline direction.
 18. A system for visual monitoring of an exposureprocess for a recording material, comprising:a process control striphaving coarse signal elements having a size substantially constant givenprocess fluctuations, and fine signal elements having a size whichchanges given process fluctuations; said control strip having a firststripe having a tonal value wedge with process-independent referencetonal values as said coarse signal elements that change in a directionof longitudinal extent of the first stripe; said control strip having asecond stripe proceeding parallel to the first stripe and having araster with fine raster points as said fine signal elements thatrepresent a substantially uniform, process-dependent tonal value; and anexposer for exposing the control strip on the recording material.